Dec. 5, 2016
Feature Story
The Stellar Side of the Exoplanet Story
K2-33b, shown in this illustration, is one of the youngest exoplanets detected to date. It makes a complete orbit around its star in about five days, and as a result, its characteristics are very much determined by its host. (NASA/JPL-Caltech)
When it comes to the study of exoplanets, it’s common knowledge that the host stars don’t get much respect.
Yes, everyone knows that there wouldn’t be exoplanets without stars, and that they serve as the essential background for exoplanet transit observations and as the wobbling object that allows for radial velocity measurements that lead to new exoplanets discoveries.
But stars in general have been seen and studied forever, while the first exoplanet was identified only 20-plus years ago. So it’s inevitable that host stars have generally take a back seat to the compelling newly-found exoplanets that orbit them.
As the field of exoplanet studies moves forward, however, and tries to answer questions about the characteristics of the planets and their odds of being habitable, the perceived importance of the host stars is on the rise.
The logic: Stars control space weather, and those conditions produce a space climate that is conducive or not so conducive to habitability and life.
Space weather consists of a variety of enormously energetic events ranging from solar wind to solar flares and coronal mass ejections, and their characteristics are defined by the size, variety and age of the star. It is often said that an exoplanet lies in a “habitable zone” if it can support some liquid water on its surface, but absent some protection from space weather it will surely be habitable in name only.
A recognition of this missing (or at least less well explored) side of the exoplanet story led to the convening of a workshop in New Orleans on “The Impact of Exoplanetary Space Weather On Climate and Habitability.”
“We’re really just starting to detect and understand the secret lives of stars,” said Vladimir Airapetian, a senior scientist at the Goddard Space Flight Center. He organized the highly interdisciplinary workshop for the Nexus for Exoplanet Space Studies (NExSS), a NASA initiative.
“What has become clear is that a star affects and actually defines the character of a planet orbiting around it,” he said. “And now we want to look at that from the point of view of astrophysicists, heliophysicists, planetary scientists and astrobiologists.”
William Moore, principal investigator for a NASA-funded team also studying how host stars affect their exoplanets, said the field was changing fast and that “trying to understand those (space weather) impacts has become an essential task in the search for habitable planets.”